1. Field of the Invention
The present invention relates to a solid electrolytic capacitor using niobium or niobium alloy for an anode and to a method of manufacturing anode material of said anode for said solid electrolytic capacitor. More particularly, a feature of the invention is to prevent increase of leakage current after heat treatment process, such as reflow soldering process.
2. Description of the Related Art
Generally, it has been proposed to fabricate a solid electrolytic capacitor by: anodizing an anode of valve metals, such as titanium, niobium, tantalum and the like or of alloy of which main component is valve metals, in phosphate solution to form a dielectric layer of oxide on the anode surface; overlaying an electrolyte layer composed of an oxide having a conductivity or a conductive polymer on the dielectric layer; and overlaying a carbon layer and a silver layer as a cathode on the electrolyte layer. (See, for example, Japanese Published Unexamined Patent Application No. 6-151258.)
However, the dielectric layer of oxide in such a solid electrolytic capacitor is easily affected by heat. Especially, the dielectric layer formed by anodizing an anode using niobium or niobium alloy is easy to be greatly affected by heat, and there has been a problem when heat treatment process, such as reflow soldering process, is conducted, a crack in the dielectric layer occurs by heat shrinkage of the anode and the electrolyte layer, and increase of leakage current is caused.
In recent years, in order to provide a solid electrolytic capacitor having little change in electric capacitance by heating of a reflow soldering process, the solid electrolytic capacitor in which a dielectric layer composed of a niobium oxide layer and a niobium nitride region is formed on the surface of an anode of niobium has been proposed. (See, for example, Japanese Published Unexamined Patent Application No. 11-329902.)
Nevertheless, a problem in the above-described solid electrolytic capacitor in which the dielectric layer composed of the niobium oxide layer and the niobium nitride region is formed on the surface of the anode of niobium has been that it is impossible to fully suppress occurrence of the crack in the dielectric layer which results in increase of leakage current.